What prop to use

[royal50]

Which prop should I use for my 94 Force 120? I believe it's a dual exhaust. I currently have a 17" pitch on it but it takes too long to get on plane and it struggles. I should also add that I like to use my boat to pull inflatable tubes, wake boarding, etc. it's on a 18" bayliner.

 

[steelespike]

You shouldn't make any prop changes without your lightly loaded wot rpm and gps speed.
Do you know your gear ratio?It is very likely you could go to a 15" pitch.
What model and hull style is the Bayliner?
Do you trim down for water sports?Do yopu try to balance the weight so not too heavy in the stern?

 

[royal50]

It's a 94 Bayliner Capri 1800. I'm not sure what the gear ratio is, honestly I'm not sure what to even look at. We we go out on the lake I do balance my weight always having someone on the bow. And I'm always trimmed down all the way. I don't think I got the trimming all figured out yet. I'm not sure where to start and how much to trim once on plane. By the way I did go to a prop chop and they suggested the 15" over my 17" because of my usage.

 

[SkiDad]

to be honest you should have zero issues with a 17p prop - something else is going on. I would expect to see at least 5000 rpm and about 38 mph with that setup. Have you verified all 4 cylinders are getting spark and fuel ?

 

[Frank Acampora]

94 Force is a 1.95 to 1 ratio lower unit. It will either be dual exhaust US Marine or thru-prop exhaust Mercury lower unit. Right around then, Mercury started using a Mercury designed lower unit on Force engines.

As stated above, I would expect good performance from a 17 pitch prop so first check that compression is high and equal on all cylinders. Check that both carbs are opening fully.

Most importantly, get a tachometer. No instrument is more important especially when wanting to change props. Without knowing RPM you are flying blind. You can't get to where you are going unless you know where you are starting.

 

[royal50]

Actually it performs decent for the most part. I say I get at least 35-38 rpm at WOT, but my tach doesn't work properly as it reads all the way down. My alarm goes off too after a few minutes but it must be a short cause all I have to do is turn my key to the left a bit and it shuts off. Last year I bought a temp sending unit and the alarm issue went away. Now the darn thing is back again. And last issue is it feels like it has a bad alignment like on a car. The boat always pulls to one side.

 

[steelespike]

There is a trim tab under the antivent plate (just above the prop).Turn the trailing end toward the direction it pulls.
​ Agreed a tach is very important for the health and performance of your motor.The motor needs to be propped so it
is able to operate within its rated rpm range at wot.
Trim all the way down as you start to speed up As the boat gets up on plane continue to trim up trim up or down for the best speed
at a given thrortrtle setting.For wot trim up till it vents ( loses grip then trim down for the best speed an grip.
Some times you may have to trim down do to weight or water conditions.Trimming down may help it to stay on plane at reduced throttle.
Is your speed by gps?Only accurate way.Assuming the speed is accurate and guessing 13% slip(guessing is bad) your rpm appears to be about 5300.
A reasonable number.
If your running wide open trimmed all the away down the boat may handle strangely.

 

[royal50]

As far as the alarm going off, any ideas?